FREETOWN, March 13 (Reuters) - Sierra Leone opposition leader Julius Maada Bio narrowly won the first round of the presidential election with 43.3 percent of the vote but fell short of the outright majority needed to avoid a second round run-off, the electoral commission said on Tuesday.

Maada Bio, who briefly ruled the West African country as head of a military junta in 1996, will square off on March 27 against first-round runner-up Samura Kamara of the ruling All People's Congress, who took 42.7 percent, in a decisive vote to replace the incumbent, Ernest Bai Koroma. (Reporting By Umaru Fofana; Writing by Aaron Ross; Editing by Kevin Liffey)